Sea ice levels shrink every summer, but this year has been different. Yesterday brought some big news: the extent of Arctic sea ice has officially reached a record low. What's more, it's done so weeks earlier than ever before — and it's not done shrinking yet.

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Before we look at the chart, let's clear something up: the measurement we're concerned with is sea ice extent, not sea ice area. Here's what the National Snow & Ice Data Center has to say about the difference between the two:

A simplified way to think of extent versus area is to imagine a slice of swiss cheese. Extent would be a measure of the edges of the slice of cheese and all of the space inside it. Area would be the measure of where there is cheese only, not including the holes. That is why if you compare extent and area in the same time period, extent is always bigger.

Extent defines a region as "ice-covered" or "not ice-covered." For each satellite data cell, the cell is said to either have ice or to have no ice, based on a threshold. The most common threshold (and the one NSIDC uses) is 15 percent, meaning that if the data cell has greater than 15 percent ice concentration, the cell is considered ice covered; less than that and it is said to be ice free.

Why report with extent instead of area? To satellite sensors, surface melt has the tendency to look like open water rather than water floating atop sea ice, "so, while reliable for measuring area most of the year, the [sensor] is prone to underestimating the actual ice concentration and area when the surface is melting [during the summer months]."

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As the NSIDC's description points out, we use satellite imagery to make these measurements, and we've been doing so since 1979. Here's the chart you need to see:

Here's what you're looking at: the dark grey line shows the average Arctic ice extent from May thru September from 1979—2000, with the grey shading around it accounting for measurement uncertainty. The dashed green line shows the Arctic sea ice extent in 2007, the previous record-holder for lowest sea-ice extent on record. The blue line shows the extent of sea ice for this summer.

The red line was added by Bad Astronomy's Phil Plait to draw attention to two important points. One: it indicates the extent of sea ice at its lowest in 2007, and illustrates that this summer, we have broken that record. Two: it reveals that we have broken that record handily. In 2007, sea ice extent bottomed out on September 9th. As of yesterday, sea ice extent for 2012 is still dropping.

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As Plait points out, the data reveals a troubling trend:

In 2011 -– which had the second-lowest extent on record, essentially equaling that of 2007 –- lowest extent happened on September 9. This year it was August 25... I don't want to make too much of the idea that it's happening earlier every year because there aren't enough data points, but it's consistent with the Earth's temperature increasing. The massive heat wave that melted so much ice in Greenland this summer may have something to do with this as well.

Where will this year's graph of Arctic ice-extent finally bottom out? We should know within a few weeks.

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Read more of Plait's analysis of the latest sea ice data over at Bad Astronomy.